A typical resonator fiber optic gyroscope (RFOG) is a rotation rate measurement apparatus that uses a recirculating ring resonant cavity to enhance the rotation-induced Sagnac effect. FIG. 1 is a typical RFOG optical circuit, which includes monochromatic light sources 111 and 112, a ring resonator cavity 100 having an optical fiber coil 118, and input and output coupling optical components 115 and 116 for coupling the monochromatic lightwaves into and out of the cavity. The coupling optical components may be optical directional couplers and/or mirrors. When the optical path of the resonator includes both optical fibers and free space, collimator lenses may be needed to couple lightwaves in the optical fiber with the optical beams in free space (not shown in FIG. 1). Other optical components, like a polarizer 117, may be used in the resonator to improve gyro performance. For rotation sensing, monochromatic lightwaves are phase- or frequency-modulated by modulators 113 and 114 before being coupled into the resonator in clockwise (CW) and counterclockwise (CCW) directions. The frequencies of CW and CCW lightwaves are tuned separately by servo electronics to the resonance frequencies of the resonator through monitoring the signals from photo detectors 121 and 122. The difference of the resonant frequencies of CW and CCW lightwaves are measured to determine the rotation rates.
The cavity of an RFOG typically supports two polarization modes that reproduce their polarization states after each round trip of the resonator. The concept of eigenstate of polarization (ESOP) has been introduced to denote these special polarization states of the resonator in analyzing the polarization error induced bias of the RFOG. For rotation sensing, the polarization state of the input lightwave is preferably aligned with one of the ESOPs of the resonator (referred to as the first ESOP) so that only one resonance characteristic is used for rotation sensing. The difference of the resonant frequencies of the CW and CCW lightwaves of this first ESOP shall be measured for determination of the rotation rate. However, due to imperfections of the alignment of the input light polarization state and polarization cross-couplings in the resonator, some light may be coupled to the second ESOP of the resonator. Since the two ESOPs have different optical path lengths in the resonator due to polarization mode dispersion, they have different resonant frequencies. Coexistence of the second ESOP with the first ESOP in the resonator causes deformation of the total resonance lineshape, leading to deviation of the measured resonant frequency from the true resonant frequency of the first ESOP. This is the cause of the so called polarization errors in the rotation sensing. Polarization induced errors may severely limit the accuracy of the RFOG.
The polarization errors in the RFOG generally depend on the magnitude of light propagating in the second ESOP. Several mechanisms may couple light into the undesired second ESOP of the resonator. Light may be cross-coupled by the resonator coupling components 115 and 116 (e.g. the couplers and/or mirrors) and the sensing fiber (optical fiber coil 118). One way to limit such polarization cross-coupling inside the sensing fiber is to employ polarization maintaining (PM) fibers. PM fiber incorporates stress elements in the fiber that define different speeds of light (i.e., birefringence) that attenuates the cross-coupling of light from one polarization state to the other. However, cross-couplings in the coupling optics will still excite the second undesirable polarization state in the PM fiber. The difference in speed of light between light traveling on the two principle axes of polarization in the PM fiber typically varies with temperature, leading to bias instability induced by polarization errors.
One method to reduce the temperature sensitivity of the light speed of the two ESOPs of the resonator is to incorporate a 90° splice 119 in the PM fiber resonator shown in FIG. 1. This effectively results in circular polarized ESOPs (one is left circular polarized, the other is right circular polarized) having 180° phase difference. The circular ESOPs have substantially identical speed of light (i.e., substantially small birefringence) and are much less sensitive to temperature variation. However, the intensity of the second ESOP is closed to that of the first ESOP in this case. Any imperfections in the 90° splice and cross-couplings in the couplers will shift the phase difference between the two circular ESOPs from 180°, generating asymmetry of the resonance lineshape used to measure rotation rate.
Another method to reduce the temperature sensitivity of the light speed of the two ESOPs of the resonator is to use a PM fiber having a hollow core. The lightwaves are mostly guided in the air core of a hollow core fiber (>95%). The birefringence of the hollow core fiber is determined by the geometric shape of the fiber cross-section instead of by the stresses. The temperature sensitivity of the fiber birefringence is significantly reduced.
FIG. 2 shows a prior art hollow core fiber resonator 200 that includes a hollow core fiber coil 210, an input coupling mirror 212, two output coupling mirrors 211 and 213, collimating optics 214 and 215 for coupling light in and out of the hollow core fiber coil 210. Use of three cavity mirrors is advantageous to guarantee all lightwaves impinging on photo detectors being spatially mode-filtered by the optical fiber. This mode-filtering reduces resonance asymmetry through removal of stray lights impinging on the photo detectors.
To further reduce the contribution of the second ESOP to the bias error, methods of inserting polarizers into the resonator and/or using polarizing fibers have also been suggested. The power in the second ESOP can be substantially reduced by the highly polarizing elements in the resonator whose pass-axis are oriented along the first ESOP.